In the field of medical engineering, there are various known systems with which, via a flexible line, liquids can be withdrawn from patients or liquids can be fed to patients. The access to the patients is generally gained in this case by means of a catheter for insertion in organs of the body or by means of a needle for puncturing vessels. During the examination or treatment, it has to be ensured that proper access exists to the patient. It is therefore necessary for the patient access to be monitored.
Proper access to the patient is also a particular prerequisite for the pieces of extra-corporeal blood treatment apparatus which have an extra-corporeal blood circuit. The known pieces of extra-corporeal blood treatment apparatus include for example pieces of dialysis apparatus and cell separators which require access to the patient's vascular system. In extra-corporeal blood treatment, blood is withdrawn from the patient along a flexible arterial line having an arterial puncturing needle and is fed back to the patient along a flexible venous line having a venous puncturing needle. In acute dialysis at intensive care stations on the other hand, what is used as a patient access is a central venous catheter in the patient's neck.
There are various known arrangements of different forms for monitoring the vascular access. These known monitoring arrangements generally rely on the safety devices which are provided as standard in the pieces of blood treatment apparatus and which trigger an immediate shutoff of the extra-corporeal blood circuit if there is not a proper vascular access.
There are known arrangements for monitoring a vascular access which have a device for detecting moisture to enable the escape of blood to be detected at the site of the puncture. These known moisture sensors take the form of a pad which has to be applied to the site of the puncture and which consists of an absorbent material to which an electrically conductive structure is applied or in which an electrically conductive structure is embedded. The electrically conductive structure comprises conductor paths arranged at a distance from one another which are arranged on or in the substrate material. Wetting of the substrate material by a liquid results in a change in the resistance between the conductor paths.
The known moisture sensors are connected by a connecting line to an arrangement which senses a change in the resistance between the conductor paths.
An electrically conductive structure which comprises two conductor paths open at the ends which are arranged at a distance from one another is a disadvantage in that the sensing arrangement is not able to detect either any damage to the conductor paths or a misconnection of the moisture sensor, because a break in a conductor path or the connecting line or the making of a faulty contact at the connection does not cause a change in resistance. It is therefore known for the ends of the two conductor paths to be connected together across a terminating resistor. If the resistance of the moisture sensor which is measured by the sensing arrangement corresponds to the resistance of the terminating resistor across the conductor paths, it can be assumed that the moisture sensor is intact and is correctly connected to the sensing arrangement.
There are known moisture sensors which consist of different substrate materials. Textile materials for example are used as substrate materials. The application of the terminating resistor to the substrate material is found to be problem in production. There are therefore known moisture sensors which do not have a terminating resistor but which are connected to an external terminating resistor. However, it is costly and complicated to connect up external terminating resistors.
International Patent Publication No. WO 2011/116943 describes a moisture sensor which takes the form of a woven material comprising non-conductive warp filaments and non-conductive weft filaments and conductive warp filaments and conductive weft filaments, which are so arranged that they form an electrically conductive structure which comprises a first and a second conductor path. The ends of the two conductor paths take the form of connecting contacts which are arranged on a tongue with a lateral offset from one another. The connector of a connecting cable by which the moisture sensor is electrically connected to the sensing arrangement is connected to the tongue. The sensing arrangement measures the electrical resistance between one pair of connecting contacts of the moisture sensor while the other pair of connecting contacts are connected together across a terminating resistor which is incorporated in the connector.
A moisture sensor which comprises an arrangement of conductor paths arranged on a substrate layer is described in German Application No. DE 10 2010 023 132. The two conductor paths of the moisture sensor are connected together by a contact layer whose resistance changes in the wetted state.
International Patent Publication No. WO 2010/091852 described a moisture sensor in which the conductor path structure and the terminating resistor are applied to a substrate material by screen printing. A printing paste containing silver is used for the conductor path structure and a printing paste containing carbon for the resistor structure. Although it is fundamentally possible for the terminating resistor to be applied by printing even in the case of a moisture sensor made of a textile material, it is found to be a disadvantage because of the additional steps in the procedure which are called for by the printing process.